(a) Field of the Invention
The present invention relates to an apparatus of driving light source for a display device such as a liquid crystal display.
(b) Description of the Related Art
Display devices used for monitors of computers and television sets include self-emitting displays such as light emitting diodes (LEDs), electroluminescences (ELs), vacuum fluorescent displays (VFDs), field emission displays (FEDs) and plasma panel displays (PDPs) and non-emitting displays such liquid crystal displays (LCDs) requiring light source.
An LCD includes two panels provided with field-generating electrodes and a liquid crystal (LC) layer with dielectric anisotropy interposed therebetween. The field-generating electrodes supplied with electric voltages generate electric field in the liquid crystal layer, and the transmittance of light passing through the panels varies depending on the strength of the applied field, which can be controlled by the applied voltages. Accordingly, desired images are obtained by adjusting the applied voltages.
The light may be emitted from a light source equipped in the LCD or may be natural light When using the equipped light source, the total brightness of the LCD screen is usually adjusted by regulating the ratio of on and off times of the light source or regulating the current through the light source.
A light device for an LCD, i.e., a backlight unit usually includes a plurality of fluorescent lamps as a light source and an inverter for driving the lamps, which includes a transformer with a boosting voltage typically determined based on the turns ratio. The inverter converts a DC (direct current) input voltage from an external device into an AC (alternating current) voltage, and then applies the voltage boosted by the transformer to the lamps to turn on the lamps and to control the brightness of the lamps in response to a luminance control signal. Furthermore, the inverter detects a voltage related to a total current flowing in the lamps and controls the voltage applied to the lamps on the basis of the detected voltage.
Since the lamp of the backlight unit is supplied with a high voltage for stable lighting operation in case of initiation or under a low temperature, the turns ratio of the transformer is set to be high to generate the high voltage.
However, since the initiation or the low temperature condition is maintained for a very short time, the lamp is not required to be supplied with such a high voltage for most time. That is, after a short-time application of the high voltage to the lamp for an initiating operation or a low temperature lighting operation, a low voltage is sufficient for driving the lamp.
In a typical backlight unit, the design of the inverter of the backlight unit focuses on the initiating condition or the low temperature condition rather than a normally operating state after ignition of the lamp. For this purpose, the ratio of the transformer is set to be high, which continuously applies high voltage to the lamp even in the stabilized state to cause unnecessary power consumption and decrease in operation efficiency.
Particularly, the efficient power consumption is very important for a device with a battery having a limited capacity such as a portable computer.